Gas turbine engines include a compression section, a combustion section and a turbine section. An annular flow path for working medium gases extends axially through these sections of the engine. Generally, air enters the compression section where it is compressed, then passes into the combustion section, where the pressurized air is mixed with a fuel (gas or liquid) and burned. The hot pressurized gases which result are then expanded through the turbine section to produce useful work, for example, by driving a generator to produce electricity or by driving a propeller in a marine propulsion system.
The overall efficiency of a gas turbine is a function of compressor and turbine efficiencies, ambient air temperature, nozzle inlet temperature and type of cycle used. Most gas turbine installations are of the open cycle type using atmospheric air as the working medium and burning relatively clean fuels such as natural gas.
Simple cycle gas turbines are relatively inefficient, with almost all losses occurring in the hot exhaust gases. When exhaust gases can be used in a boiler or for process heating, the combination of a turbine with a heat recovery apparatus results in a high efficiency power plant. Another method that results in high efficiency is to integrate the gas turbine with other process requirements.
For maximum efficiency, the turbine section must operate at the highest temperatures possible. However, high temperatures have a negative impact on turbine life. Therefore, to balance these two factors, cooling air is usually injected into the turbine, with the air flowing inside the turbine blades and vanes to cool them while they are in contact with the hot combustion gases. This air is obtained by taking a side-stream of compressed air and injecting it into the turbine section.
In land-based gas turbines, there has been a continuing trend towards improving thermal efficiency while reducing noxious emissions. One idea for improving efficiency involves precooling the turbine cooling air prior to its entry into the turbine. Such cooling increases the density of the air and increases the temperature differential, reducing the amount of cooling air needed to meet turbine cooling requirements. This reduces a loss to the cycle, by increasing the amount of compressed air which passes into the combustor, improving overall efficiency. Typically, this cooling air is obtained by passing the compressed air through a fan cooled heat exchanger for rejecting the heat of compression to the atmosphere.
To meet emission requirements for noxious gases such as nitric oxides (NOx) produced in the combustion cycle, water or steam is injected into the combustor, quenching the hottest combustion zones. Preventing a wide temperature gradient in the combustor would also minimize nitric oxide formation.
In liquid fueled turbines which produce steam by heating water with the hot exhaust gas, separate steam injection into the combustor is typically used for nitric oxide control. Water may be used with liquid fuels, by mixing with the fuel prior to injection in the combustor.
With gaseous fuels such as natural gas, steam is directly injected into the fuel gas, avoiding a separate steam injection manifold. Although steam can be injected separately into the combustor, NOx control is improved if the steam is premixed with the fuel, thereby avoiding hot spots due to insufficient steam/fuel interaction in the combustor. However, the gaseous fuel must be heated prior to mixing to avoid injecting a slug of liquid, i.e. steam condensate, into the combustor, which would cause instabilities in the combustion process or high thermal stress in the combustion chamber.
Part of the fuel heating may be accomplished by directing cooling air exiting the gas turbine enclosure to interchange through a heat exchanger with the entering fuel thereby preheating the fuel up to about 70.degree. C. However, a relatively large heat exchanger is required as the cooling air is at atmospheric pressure and is relatively cool. In addition, further heating, up to about 120.degree. C., must be accomplished by a second heat exchanger to prevent condensation. Therefore, low pressure steam must additionally be used, a thermal energy loss, requiring a separate heat exchanger with associated piping. Alternatively this heating could be done with heat from the turbine exhaust gas, but this reduces the heat available for producing steam.
Another alternative is to use highly superheated steam to heat the steam pipes, thus allowing a degree of cooling without condensation. However, this similarly reduces efficiency and continues the risk of water condensation after mixing with the fuel gas.